Titanium dioxide is the most common white pigment due to its strong ability to backscatter visible light, which is in turn dependent on its refractive index. Substitutes for titanium dioxide have been sought, but the refractive indexes of both the anatase and rutile forms of this oxide are much higher than those of any other white powder, due to structural reasons.
Titanium dioxide pigments are insoluble in coating vehicles in which they are dispersed. The performance properties of such titanium dioxide pigments, including its physical and chemical characteristics, are determined by the particle size of the pigment and the chemical composition of its surface. Titanium dioxide is commercially available in two crystal structures: anatase and rutile. Rutile titanium dioxide pigments are preferred as they scatter light more efficiently and are more stable and durable than anatase pigments. Titanium dioxide scatters light very efficiently due to its large refractive index. The decorative and functional attributes of titanium dioxide, due to its scattering capabilities, make it a highly desirable pigment. However, titanium dioxide is known to be an expensive pigment to manufacture. Accordingly, there is a need for a more affordable substitute for titanium dioxide as a pigment.
As mentioned, a desired feature of titanium dioxide is its large capacity of spreading (or scattering) the visible light. This property is the result of its high refraction index, together with the absence of electronic transitions in the visible part of the spectrum. Many attempts have been carried out to replace the titanium dioxide, partially or totally in its applications as pigment. However, the refraction indices of its two forms, anatase and rutile, are difficult to obtain by other white solid substances (Handbook of Chemistry and Physics, CRC Press, 57th ed., 1983). Thus, the search for new white pigments led to the search of systems with other light spreading mechanism. Multiphase media, which present a large variation of the refraction index, may operate as light spreaders.
The current options for manufacturing processes of pigments or paints that result in a film containing “pores” in the internal part of the particles or between the particles and the resin is also quite limited. Some techniques for hollow particle preparation have been described in the literature, however, most techniques involve the manufacturing of spheroidal hollow and polymeric particles by polymerization in emulsion. An example is the study of N. Kawahashi and E. Matijevic (Preparation of Hollow Spherical Particle of Itrium Compounds, J Colloid and Interface Science 143(1), 103, 1991) on the coating of the polystyrene latex with basic itrium carbonate and subsequent calcination in high air temperatures, producing hollow particles of itrium compounds.
The preparation of hollow particles of aluminum metaphosphates by chemical reaction between the sodium metaphosphate and aluminum sulfate, followed by thermal treatment, was described by Galembeck et al. in Brazilian Patent BR 9104581. This study referred to the formation of hollow particles of aluminum phosphate synthesized from sodium phosphate and aluminum nitrate. As mentioned, the two pigments, aluminum phosphate and metaphosphate, can be used to replace a large part of TiO2 in paints based on PVA latex or acrylic emulsions.
Brazilian Patent BR 9500522-6 of Galembeck et al. describes a way of making a white pigment from a double aluminum and calcium metaphosphate, obtained directly by a chemical reaction between the aluminum metaphosphate and calcium carbonate particles in a polymeric latex emulsion type aqueous medium. This patent extended the previous results to calcium salts that, from the environmental point of view, are advantageous due to their full atoxicity.
Several publications discuss the synthesis of aluminum phosphate materials primarily for use as a catalyst support including crystalline and amorphous forms. Many of these methods yield highly porous and crystalline forms and few thermally stable amorphous compositions. Examples of such materials are described in U.S. Pat. Nos. 3,943,231; 4,289,863; 5,030,431; 5,292,701; 5,496,529; 5,552,361; 5,698,758; 5,707,442; 6,022,513; and 6,461,415. There exists a need, however, for aluminum phosphate with hollow particles, particularly for a powder that could be manufactured with relative ease.